This invention relates to a method and an apparatus for processing tobacco.
In the tobacco industry, it is well-known that in order to process the tobacco into a suitable form for use in the manufacturing of products, the tobacco leaf has to have the midrib stem removed from the rest of the tobacco leaf (hereinafter referred to as the lamina). This process is generally referred to as threshing.
In the current art, the most common leaf threshing process consists of:
1. Feeding the tobacco leaves into the top of a threshing mill. The leaves are broken up by the action of a rotating element, with radially protruding teeth, until they are small enough to pass through a fixed screen at the bottom of the mill.
2. The threshed leaf is then classified using an air flow in a vertical tower. The lighter, stem free, material rises with the air flow and is removed from the threshing process. The heavier, stem-containing material drops, under the influence of gravity, down the tower, through the air.
3. This heavy, stem containing, material is passed to a second threshing mill, and the process is repeated.
The overall process usually has between four and six stages of threshing and classifying before all the lamina is removed from the midrib stem.
At each stage the aperture size of the screen at the bottom of the mill reduces.
This process is illustrated by means of the flow diagram in FIG. 1.
It can be observed from this description that it is not possible to reduce the number of threshing stages in use in a process of this form by using a recycle system. The heavy stems would have no means of escaping from the recycle and would accumulate in the process, rapidly overloading it.
GB 740088, U.S. Pat. No. 2,697,439, U.S. Pat. No. 3,046,998 and U.S. Pat. No. 3,513,858 each describe methods of recycling heavy stems with lamina still attached. in each of the processes, an air classification system is used to separate the tobacco into three streams i.e., lamina, clean stems and stems with lamina still attached. The three stream air classification systems are relatively complicated.
U.S. Pat. No. 3,661,159 describes apparatus for stemming tobacco leaves which separates the threshed leaves into light and heavy fractions.
Classification of threshed tobacco leaves into light and heavy fractions is also disclosed in EP-A-0707800, GB-A-2157411 and WO90/05034. The apparatus mentioned in WO90/05034 includes a sieve to separate out the largest size particles after threshing but deals only with improvements in classification systems.
The present invention solves the problem of allowing the use of a recycle system in the processing of tobacco leaves without the need for a complicated three stream air classification system.
According to the present invention, there is provided a method for processing tobacco comprising threshing tobacco leaves in a thresher to form a mixture of free lamina, clean stem pieces from which lamina has been completely or substantially completely removed and unclean stem pieces which have useful pieces of lamina attached thereto, sifting to separate the clean stem pieces from the unclean stem pieces and subjecting the unclean stem pieces to further threshing by recycling the unclean stem pieces to the thresher. The unclean stem pieces are recycled to the same thresher in which they were first produced by threshing tobacco leaves.
The separation of the clean stem pieces from the unclean stem pieces may be carried out at any stage after the threshing step e.g., before or after the conventional separation (or classification) of the lamina from the stem-containing material.
Therefore, in one preferred embodiment, the free lamina is removed from the mixture before sifting. The term sifting is used synonymously with the term sieving in this specification.
In another preferred embodiment, the mixture of lamina, clean stem pieces and unclean stem pieces is sifted to separate the clean stem pieces, and the free lamina is then removed from the unclean stem pieces which are later subjected to additional threshing.
The threshing of the tobacco leaves may be carried out by conventional methods well-known in the art. Sifting of the threshed tobacco may be carried out separately from the threshing step. Alternatively, threshing and sifting may be carried out together in a single piece of apparatus. Such an apparatus for threshing and sifting is described in our related European patent application no. 96309198.8 which is entitled xe2x80x9cApparatus and Process for Threshing Tobaccoxe2x80x9d and has the same filing date as this application.
Preferably, the free lamina is removed from the mixture or the unclean stem pieces by air classification which is a conventional technique well-known to those skilled in the art.
Sifting to separate the clean stem pieces from the unclean stem pieces (either mixed with or without free lamina pieces) is carried out by the use of a sieve or a similar device. The sieve can be of any type known within the art. Preferably it is a cascade type or rotary type with wiped sides, which are less prone to malfunctioning as a result of screen blocking. The screen size used in the sieve will depend on the size and type of leaves being processed, but commonly the aperture size in the screen will be between 10 and 100 mm.
The type of screen used in the sieve can be of any type known within the art.
The tobacco entering the process may optionally be cut into sections across the stem before threshing, usually referred to as tipping or butting, and the unclean stems may be cut into shorter lengths prior to being threshed with unthreshed tobacco leaves. The shorter stem length improves the sieving of the clean stems after they have been threshed.
The invention also provides apparatus for processing tobacco comprising threshing means for threshing tobacco leaves to form a mixture of free lamina, clean stem pieces from which lamina has been completely or substantially completely removed and unclean stem pieces which have useful pieces of lamina attached thereto, wherein the apparatus further comprises means for sifting to separate the clean stem pieces from the unclean stem pieces and means for returning the sifted unclean stem pieces back to the threshing means. The means for sifting is preferably a sieve. The means for returning the sifted unclean stem pieces back to the threshing means may comprise a conventional conveying device such as a conveyor belt.
By the term clean stem pieces, it is meant that the stem pieces are not attached to useful pieces of lamina i.e., pieces of lamina which could be relatively readily separated from the stem in a threshing process. Similarly, the term unclean stem pieces refers to pieces of stem which do have useful pieces of lamina attached.
The nature of the clean stem pieces will depend upon the aperture size of the screen used in the sieve. Where the aperture size is low, the amount of lamina attached to the stems passing through the screen will be small, typically less than 1% of the total lamina entering the process. Where the aperture size is large, the amount of lamina attached to the stems passing through the screen will be greater, up to 20% of the total lamina entering the process, but is preferably not more than 10% of the total lamina entering the process.
Where the stems produced are clean or contain a very small amount of attached lamina, they can be removed from the threshing process and passed without further processing to a drying stage or other process.
Where the amount of lamina still attached to the stems is greater or where totally clean stems are required for drying or further processing they can be passed first to a final polishing process. The final polishing process can consist of any type of suitable threshing process known within the art. Preferably it consists of a single conventional thresher and classifier or a further single conventional thresher, classifier and sieve with a small aperture screen as described in this application, or carried out using the apparatus described in our related application no. 96309198.8.
The unclean stems i.e., the stems which are still attached to relatively large pieces of lamina, can be recycled to any of the previous threshing stages.
The main advantages of the present invention are as follows:
1) The number of unit processes in a threshing plant can be reduced.
2) The average size of lamina produced from the process is larger.
3) Less stem fibres and dust are produced in the process.
The first of these advantages arises from the recycle process using the preferred apparatus of the invention where over 90% of the lamina can be removed in the first threshing stage, compared to less than 70% in a conventional single pass process.
The second of these advantages results from the fact that the invention allows recycling of the stems with large pieces of lamina attached, into a thresher with a large aperture screen at the bottom of the mill. The large pieces of lamina which are freed from the stems do not then have to be broken further in order to escape through the screen.
The third of the benefits arises from the effect of the process in reducing the number of times that the stems have to pass through a thresher. The majority of stems will pass through a thresher only twice, or three times where a final polishing stage is used, instead of the four to six times the stems pass through a thresher in a conventional threshing process.
The threshing means and means for sifting are preferably provided in a single combined threshing and sifting device, as disclosed in European patent application no. 96309198.8. The device comprises rotatable stripping means and a feeder comprising a pair of co-operating counter-rotatable elements for delivering tobacco leaves to the stripping means, the feeder and the stripping means being arranged such that, in use, the tobacco leaves experience shearing forces as they pass from between the counter-rotating elements to the rotating stripping means which forces at least partially strip the lamina from the stem.
The stripping means preferably comprises radially extending arms. The arms may be in the form of continuous plates or spaced teeth (which can be straight, bent or curved) and may include parts which are capable of cutting the tobacco leaves or are capable of puncturing the leaf and tearing through the leaf.
The elements also preferably comprise radially extending arms which also may be in the form of straight, curved or bent continuous plates or spaced teeth, optionally including parts which are capable of cutting the tobacco leaves. Alternatively, the feeder elements may comprise a pair of rollers or may consist of a flexible membrane wrapped around a framework which allows variable quantities of leaf to be fed.
The arms may be flexible, inflexible or a mixture of fixed and flexible components.
The rotating elements and the stripping means can take the same form and may be the same shape and size, differing only in their function which is dictated by their position in the device.
The stripping means and the feeder are arranged at a relatively close distance from each other in order to subject the tobacco to the shearing forces which at least partially strip the lamina from the stem. Preferably, the distance between the outermost parts of the two elements in the feeder is less than 150 mm and the distance between the outermost parts of each of these elements and the outermost part of the stripping means is less than 100 mm. The shearing forces are experienced by the tobacco leaves as they are directed from the feeder to the rotating stripping means. The tobacco leaves are preferably delivered to the rotating stripping means substantially along a radius of the axis of rotation of the stripping means. The lamina is stripped from the stem in the region where the leaves pass from being moved under the influence of the feeder to being moved by the stripping means and, as the skilled person will appreciate, the exact position of this region will vary depending upon the particular configuration of the device and the rate of rotation of its various rotating components. The shearing force can be considered, at least in certain circumstances, as arising from the action of the stripping means on one part of the leaf while the feeder is holding another part of the leaf.
Preferably, where the arms of the stripping means and those of the elements consist of spaced teeth, the teeth are intermeshed. With such an arrangement, the stripping of the lamina from the stem is effected, to some extent, by the interaction of the stripping means and the feeder. When the teeth are intermeshed, the distance between the teeth as they pass each other is typically less than 100 mm.
The device preferably comprises a rotatable drum in which the stripping means and the feeder are arranged. The drum comprises a screen (e.g., in its walls) which allows the clean stem pieces to be sieved and to pass out of the drum. The screen can form all or only part of the side walls of the drum.
The drum preferably incorporates internally protruding lugs for directing the tobacco leaves to the feeder elements. As the drum rotates, the lugs collect the tobacco leaves so that the leaves travel around the inside of the drum until they fall (preferably solely under the influence of gravity) into the feeder. The positioning of the feeder within the drum and/or the speed of rotation of the drum are adjusted so as to ensure that a suitable amount of the tobacco leaves is delivered to the feeder as the drum rotates.
The stripping means and the elements preferably rotate about axes which are parallel to the axis of rotation of the drum. Preferably, the axes of rotation are either substantially horizontal or are tilted at from 0 to 10xc2x0 (more preferably 2xc2x0 to 6xc2x0) to the horizontal.
The device can comprise more than one (e.g., two) stripping means and the extra stripping means can be provided by one or more additional rotating elements. These additional elements may carry out a degree of threshing and/or cutting of the tobacco leaves and they may participate in the delivery of the partially threshed or unthreshed leaves to the feeder and/or the stripping means. The device may also comprise additional rotating elements which act solely to direct leaf material into the stripping means.
The stripping means and elements can converge as they approach the exit end of the drum to take account of reduced loading resulting from the small particles being sieved out of the main flow. This also allows the amount of threshing that takes place to increase towards the exit end of the drum. This convergence can be achieved by varying the diameter of the stripping means and/or the elements along their length. By adopting this method, an additional effect will be to increase the tangential velocity of the arm tips as the diameter increases and this will affect the threshing characteristics.
The diameters of the stripping means and the elements (as defined by the end of any radially protruding arms) are typically from 5 to 50% of the diameter of the drum.
The tangential velocity of the drum is preferably in the range of from 0.04 to 0.5 m/s with the tangential velocity of the stripping means and the elements being from 3 to 250 times (preferably 10 to 100 times) greater than that of the drum.
The rotating components of the device (other than the drum) can be run at synchronous speeds to achieve true intermeshing of the stripping means and the rotating elements. Alternatively, the rotating components can be run such that they have different tangential velocities, thus setting up shear actions between the various rotating components. The choice of rotational speed and mode of operation depend upon the design of the rotating component and drum protrusions used and this in turn depends on the nature of the leaves being threshed and upon the required specification of the threshed material.